What is the effective range of an ADS-B signal?

Range is a function of radio line-of-sight and is dependent on antenna height, aircraft altitude and terrain.

Air-to-Air: 170 - 200 nautical miles (270 - 320 Km) for UAT.

Air-to-Ground: 180 nautical miles (288 Km) for high altitude targets .

Although the theoretical maximum ranges for UAT and 1090 ES are quite similar, in practice, the effective range for 1090 ES can be somewhat less due to bandwidth congestion.